The potential use of lasers interfaced with optical wave guides and incorporated into catheter systems is a provocative potentially important area of future clinical investigation. However, before such bold instrumentation can be employed in humans, a variety of basic studies helping to clarify correct laser sources and to predict biological effects of laser-tissue interactions are necessary. The present investigation includes an evaluation of four different lasers (Argon, Neodymium-YAG, Carbon Dioxide, and Ultraviolet Excimer) with widely varying physical properties studied during conditions of altered power and pulse duration. Laser effects were studied in dry medium (air) and in solution (saline), with and without appropriate optical wave guides, and tissue specimens employed were fresh human cadaver coronary arteries in both normal patients and patients with known underlying atherosclerosis. Specific laser effects were assessed using thermocouples and fast infrared photography to define thermal diffusion properties and gross pathologic and light microscopic changes. These experiments have helped to differentiate specific effects of given laser sources and will provide important information to be used in subsequent in vivo animal studies.